Can your HRV Number be too High? Here’s the Research

Who, What and Why

Back in 2009 we had a report from one of the original ithlete beta testers that during an ultra endurance event he got an unusually high waking HRV, despite feeling totally fatigued. We have been intrigued ever since by the notion that the balance between sympathetic and parasympathetic can shift too far towards the latter, which is of course the opposite direction to what most people expect when too high stress levels have taken their toll.

Most of the research has focused on the more commonly occurring reduction in heart rate variability (HRV) with increasing training loads and stress, and good quality studies on parasympathetic overreaching have been lacking, but recently, two studies from sports science infographic supremo Yann Le Meur at the French National Institute for Sport shed more light on how & when this form of overreaching occurs.

What Did They Do?

In the first study, 24 well trained triathletes (who trained at least 6 times per week) took part in a 7 week study and were separated into two groups. Eight of them acted as a control, and performed their usual training, whilst the other 16 volunteered to have their usual training load increased by 40% for a period of 3 weeks in the middle of the study period. Both the session durations and the number of high intensity repetitions were increased to deliberately induce overreaching. Training load was monitored using time in each of 3 zones (Lucia TRIMP, as used in Precision Pulse), and the zone thresholds were also re-measured during the study in case they had shifted.

HRV was measured every morning in both lying and standing positions for the 3 intensified weeks and the taper week either side. RMSSD and resting HR as well as HF & LF frequency domain parameters were calculated. Subjective tiredness was also recorded once per week using a 0-100 fatigue scale. A weekly maximal running test on the track was used to verify overreaching i.e. that performance was significantly worse than that of the control group in spite of continued training.

In the second study, the researchers decided to use a parameter called heart rate recovery (HRR), which is measured at the end of a standardised exercise, and is often considered to reflect progress in overall fitness. Basically, if an athlete’s heart rate recovers more quickly after stopping exercise, they are considered to be fitter than they were previously. The rate at which HR decreases after exercise depends on the rate of sympathetic withdrawal, and parasympathetic reactivation, so this additional measure of ANS balance could give additional insight as to the mechanisms behind reduced performance whilst in an overreached state. They also studied well-trained male triathletes before, during and after a 3-week overload period. HRR and blood hormone levels were measured after a maximal cycling test performed before and after the 3-week overload period.

What Did They Find?

They found that 15 of the 16 intensified training group (F-OR in the figure) showed decreased performance after the overload period, so the overreaching had been successful in all but one case (whose data was then excluded from further analysis). Performance then rebounded after the rest week, proving that the overreaching was functional, whereas recovery from non-functional overreaching takes several weeks or even months.

The heart rate measures demonstrated consistent patterns, but with varying degrees of significance:

Resting heart rate (RHR) decreased during the overload period.

The decrease was more progressive and significant in the standing rather than lying position

Significance was greater using weekly moving averages than single day values

RHR increased back towards, but did not reach pre-overload levels after the rest week

Time domain HRV (LnRMSSD) increased during the overload period

Single day isolated measures only achieved significance after the third week, irrespective of body position

Weekly averaged values reached significance after the first week

The standing position appeared to give the greatest sensitivity, especially in respect of the decrease during the post-overload rest week

What was also notable from this study was the heart rates at low intensity and threshold, which also decreased very significantly during the overload period.

In the second study, the researchers found that many (though not all) of the athletes in the overload group met the criteria for overreaching (f-OR) after the overload period, and that their heart rate recovery increased very significantly, reducing again during the subsequent rest week. The remainder of the overload group (AF) also showed a likely increase in HRR, but as the diagram shows this was less significant:

This increase in HRR was therefore associated with reduced performance, rather than improved performance, its more usual interpretation. What’s also very interesting is the decrease in peak exercise heart rate (B.) and epinephrine (adrenaline, D.) concentration in the overreached group:

What does it mean?

The first study clearly demonstrated trends in resting HR and HRV that are opposite to those often expected and experienced by recreational athletes. The study group were relatively serious athletes, training 10-12hr per week regularly, and achieving times of 2:00 to 2:20 in Olympic distance events. Training time was increased to almost 17 hours per week during the overload period, which is needless to say a big increase on the load and well beyond the more usual 10% per week increase, which most athletes can tolerate.The second study also clearly showed a relative parasympathetic dominance using the HRR measure. This study also measured blood adrenaline levels significantly reduced in the overreached state.

Despite the very clear results, the mechanisms behind these alterations are not fully understood yet, and the authors suggest can be caused by a combination of 3 effects:

A decrease in central command – ie less internal stimulation during intensive exercise

Rather than always interpreting HRV rises as a good thing, we recommend putting HRV rises into context as follows:

In a fast-rising HRV trend situation, look at resting HR as well. If it is decreasing fast, you may be heading towards this form of overreaching. The ithlete mobile app includes colour coded indicators for these trends, but the ithlete Pro Training Guide allows them to be followed more precisely, and using a standardized scale of standard deviation units, which are comparable between individuals and over time:

If using heart rate recovery (HRR) as a post exercise measure, look out again for fast rising trends over a few days or a week. Aerobic fitness does not increase that quickly, but takes several weeks

Record subjective mood and fatigue indications and be honest with yourself – if you feel tired and a little demotivated, record it that way. We wrote a full post about tracking your subjectives here.

Record Training Load and calculate a weekly average. If this is increasing more than 10% per week, then overreaching is a distinct possibility. The Precision Pulse app can record and calculate the 3-zone TRIMP used in these studies and can also be used to calculate HRR manually at the end of exercise.

What I have found in my experience is that the high HRV rebound reflects an immune over compensation via activation of a side of the immune system that deals with anti-inflammatory effect (i.e. IL-10 etc…). Often this is also accompanied by a very different value from supine to standing compared to normal. Yet, according to my research the exact mechanisms of this process are not fully understood. As mention in this article if the HRV is quite high as a rebound effect in response to high load (both timely and intensity) I normally suggest to keep the recovery at least until it has stabilised back in average ranges.

Thanks for the observations Alex. As Alan Couzens observed on Twitter relating also to this research – ‘Just like 3 bears – not too high, not too low. Best training comes when HRV is just right.’
…and they all lived happily ever after with the ithlete Pro Training Guide!

Highly interesting, I’m an amateur cyclist just having completed a full months cycling in Mallorca. I was noticing this a lot today, difficulty in achieving threshold heart rate, some 15-20 bpm off and seeing my heart rate fall much lower than i would normally when well rested. The body was certainly not willing today!

Thanks for the feedback, Rob. Although a lot of people have noticed this effect, it’s still not fully explained by science. Whether it’s due to a fall in adrenaline production, or a protective downgrade in sensitivity to adrenal hormones, or something else, no-one seems quite sure. I’ve been there, and can also testify that ‘the body is not willing’! Hope you enjoyed Mallorca – I just signed up for the 312 again next year. Simon.

Simon THANK YOU. Not sure why I only saw this now, but your articles are always extremely useful and interesting. The body is indeed a complex mechanism. I think it also helps tremendously that you are not just a scientist but a practising athlete who can speak from experience.

Thank you Sonja for your ongoing support! I do continue to find the complexities of the body fascinating, and often ponder how the various mechanisms evolved to be like they are today.
We have quite a few articles in the bank and plan to organise them better for users and anyone interested in HRV to access.